hybrids were decidedly more fertile than either of their pure parents. There is no way to escape from the admission that the hybrids from some species of plants are fertile, except by declaring that no form shall be considered as a species, if it produces with another species fertile offspring: but this is begging the question{246}. It has often been stated that different species of animals have a sexual repugnance towards each other; I can find no evidence of this; it appears as if they merely did not excite each others passions. I do not believe that in this respect there is any essential distinction between animals and plants; and in the latter there cannot be a feeling of repugnance. {243} <Note in the original.> If domestic animals are descended from several species and _become_ fertile _inter se_, then one can see they gain fertility by becoming adapted to new conditions and certainly domestic animals can withstand changes of climate without loss of fertility in an astonishing manner. {244} See Suchetet, _L'Hybridite dans la Nature_, Bruxelles, 1888, p. 67. In _Var. under Dom._, Ed. ii. vol. II. hybrids between the fowl and the pheasant are mentioned. I can give no information on the other cases. {245} _Origin_, Ed. i. p. 250, vi. p. 370. {246} This was the position of Gaertner and of Koelreuter: see _Origin_, Ed. i. pp. 246-7, vi. pp. 367-8. _Causes of Sterility in Hybrids._ The difference in nature between species which causes the greater or lesser degree of sterility in their offspring appears, according to Herbert and Koelreuter, to be connected much less with external form, size, or structure, than with constitutional peculiarities; by which is meant their adaptation to different climates, food and situation, &c.: these peculiarities of constitution probably affect the entire frame, and no one part in particular{247}. {247} <Note in the original.> Yet this seems introductory to the case of the heaths and crocuses above mentioned. <Herbert observed that crocus does not set seed if transplanted before pollination, but that such treatment after pollination has no sterilising effect. (_Var. under Dom._, Ed. ii. vol. II. p. 148.) On the same page is a mention of the Ericaceae being subject to contabescence of the anthers. For _Crinum_ see _Origin_, Ed. i. p. 250: for _Rhododenron_ and _Calceolaria_ see p. 251.>